SummaryBALB/c mice develop resistance to challenge withN. dubiusthird stage infective larvae (L3) 2–3 weeks after the administration ofN. dubiusworms. In contrast, CBA/H mice fail to develop resistance. Experiments were performed to test the hyptothesis that the reason for the different behaviour of these mice was a difference in their eosinophil response. BALB/c mice and BALB/c → (BALB/c → CBA/H)F1bone marrow chimaeras but not CBA/H mice or CBA/H → (BALB/c × CBA/H)F1chimaeras mounted strong eosinophilia afterN. dubiussensitization and the eosinophil response Was associated with resistance to L3 infection in these mice. Induction of a mild eosinophilia in CBA/H mice by non‐parasite antigen keyhole limpet haemocyanin (KLH) in complete Freund's adjuvant (CFA) following cyclophosphamide (CY) treatment was associated with the acquisition of a degree of resistance to L3 infection. In BALB/c mice, increase in eosinophilia induced by CY and KLH‐CFA was not associated with a further reduction in worm numbers. These experiments support the hypothesis that eosinophilia is an important component of host protective immunityin vivo, and suggest a new basis for the heterogeneity of the outcome of some parasite infestations.